The present invention relates to an eccentric oscillating-type planetary gear device that causes eccentric oscillation of an externally toothed gear meshed with an internally toothed gear by a crank shaft.
An eccentric oscillating-type planetary gear device, for example, one disclosed in Patent Document 1 is known.
The known eccentric oscillating-type planetary gear device includes an internally toothed gear, in which internal teeth having a plurality of rod-shaped pins on an inner periphery are provided by a constant pitch, an externally toothed gear, in which a plurality of crank shaft holes and through holes are formed and which has external teeth meshed with the internal teeth on an outer periphery, the external teeth having a trochoid tooth profile and being less than the internal teeth by one, a crank shaft that is inserted into each of the crank shaft holes and rotates to cause eccentric oscillation of the externally toothed gear, and a support body that rotatably supports the crank shaft and has a plurality of pillar portions respectively loosely fitted into the through holes.
In the known eccentric oscillating-type planetary gear device, as shown in FIG. 22, drive force components in a direction perpendicular to tooth surfaces are correspondingly given from external teeth 02 of an externally toothed gear 01 to internal teeth (pins) 04 of an internally toothed gear 03 at contact points where the externally toothed gear and the internally toothed gear are brought into contact with each other. Then, as the reaction, reaction force K of the drive force components are correspondingly given from the internal teeth (pins) 04 to the external teeth 02.
In addition, as shown in FIG. 23, action lines S of reaction force K of drive force components correspondingly given from the external teeth 101 to the internal teeth (pins) 102 meet at one meeting point C. The meeting point C is positioned between an outer end passing circle G passing radially outer ends of all through holes 103 and an inner end passing circle N passing radially inner ends of the through holes.
In such a planetary gear device, it is necessary to increase output torque without making the device large in size, in particular, without making the externally toothed gear large in diameter, and without increasing the drive force components (contact pressure) correspondingly given from the external teeth to the internal teeth.
Here, the output torque becomes the sum of values obtained by multiplying a tangential component of the drive force component at each of the contact points of the external teeth and the internal teeth (pins) by a distance from the center of the internally toothed gear to the contact point. However, the distance from the center of the internally toothed gear to the contact point is constant in order to prevent the device from becoming large in size. For this reason, in order to increase the output torque, an increase in the tangential component of the drive force component is taken into account. Then, the increase in the tangential component of the drive force component can be achieved by moving a meeting point where the action lines of the drive force components meet radially outward and by inclining the action lines toward a tangential direction with respect to the externally toothed gear.
Patent Document 1: JP-A-7-299791
In the above-described planetary gear device, however, the thickness of each of bridge portions 06 positioned radially outside the through hole 05 in the externally toothed gear 01 (a radial distance from the radially outer end of each of the through holes 05 to the tooth bottom 07 of each of the external teeth 02 becomes the minimum thickness) is slightly smaller than the thickness of other portion, and thus bending rigidity is low. Accordingly, the reaction force K described above is applied to the bridge portions 06 in an approximately radial direction, the bridge portions 06 and the external teeth 02 close to the bridge portions 06 are elastically deformed, and the external teeth 02 and the internal teeth (pins) 04 are unevenly meshed, which causes the short life of tooth surfaces of the external teeth 02.
Besides, as described above, if the bending rigidity of the bridge portions 06 is low, when a torque load exists in a case where the planetary gear device is applied to a robot, a machine tool, or the like, a natural vibration frequency is low, and thus vibration characteristics are degraded. In addition, controllability is decreased.
In such a known eccentric oscillating-type planetary gear device, as described above, since the meeting point C is positioned between the outer end passing circle G and the inner end passing circle N, with eccentric oscillation and rotation of the externally toothed gear 104, the meeting point C is positioned close to the center of the through hole 103. Then, the action lines S of the reaction force K extend in an approximately normal direction with respect to the through hole 103. Here, the bridge portions 105 positioned radially outside the through hole 103 in the externally toothed gear 104 are thinner than other portions, and thus rigidity is low. Then, a part of the reaction force K is applied to the bridge portions 105 having low rigidity in the approximately normal direction of the through hole 103, that is, in a direction approximately perpendicular to an extension direction of the bridge portions 105, as described above. Accordingly, the bridge portions 105 and the external teeth 101 close to the bridge portions 105 are elastically deformed, and the external teeth 101 and the internal teeth (pins) 102 are unevenly meshed, which causes the short life of the tooth surfaces of the external teeth 101.
The amount of eccentricity of the externally toothed gear to the internally toothed gear, which is obtained by dividing the position of the above-described meeting point (the radial distance from the center of the internally toothed gear) by the number of internal teeth (pins), needs to be less than 0.5 times the radius of each of the internal teeth (pins) in order to prevent the tooth tops of the external teeth from interfering with the inner periphery of the internally toothed gear (in the related art, about 0.40 to 0.45 times). As a result, as described above, the movement of the meeting point radially outward is limited, and the output torque cannot be sufficiently increased.